This disclosure generally relates to mask inspection, and more specifically to inspecting an alternating phase shift mask using biased inspection data.
Currently, state of the art semiconductor manufacturing utilizes 193 nm wavelength optical lithography systems in conjunction with phase shift masks such as alternating phase shift masks to create circuit patterns. A problem that alternating phase shift mask manufacturers face is that currently available mask inspection tools used to find defects in the masks after fabrication utilize a non-actinic wavelength of light which does not match with the 193 nm wavelength optical lithography systems. When there is a mismatch between the wavelength of light used to expose an alternating phase shift mask and the wavelength of light used to inspect the mask, a die-to-database inspection methodology will generate a very high number of false defect detections when inspecting an alternating phase shift mask. A die-to-database inspection methodology, which compares images taken from the alternating phase shift mask to images of the design data used to fabricate the mask, will generate a high number of false detection of defects because the images observed for the unetched (0 degree regions) and etched regions (180 degree phase shift) of the mask will differ from the size of the stored images of the design data due to the variation in the wavelength of light used to expose the mask during circuit fabrication and that used to inspect the mask during mask fabrication. With such a high rate of false detection of defects, mask manufacturers typically have to reduce the sensitivity of the inspection of the alternating phase shift mask to quartz defects (etched regions).
These mismatch problems are not limited solely to situations when a mask is exposed on 193 nm wavelength optical lithography systems, but are inherent in any situation where the wavelength of light used to expose the alternating phase shift mask in the circuit fabrication process is different from the wavelength used to inspect the alternating phase shift mask during its fabrication process.
One way to avoid the mismatch problems is to use a die-to-die inspection methodology which compares two identical images on the mask having the same pattern and identifies any discrepancies between the two. In this methodology, the difference in the wavelength of light used to expose the mask during circuit fabrication and that used during mask inspection is not an issue because the die-to-die inspection is based only on the wavelength of light used to inspect the mask during its fabrication. Unfortunately, a die-to-die inspection methodology is not practical because many of today's alternating phase shift mask designs are too large to have a copy of the design on the same mask.